A thin high-doping layer was inserted in the uniform doped collector to extend the operational current before current gain and cut-off frequency roll-off. Two times higher collector current before onset of Kirk effect was obtained and the resulted John figure of merit was improved from 846 to 1008 V-GHz.

In this paper, we report a new collector-up npn heterojunction bipolar transistor (C-up HBT) which employs a p-type doping buried layer inserted between extrinsic emitter and subemitter for current confinement. A theoretical study is performed to verify the functionality of the p-type doping buried layer using a two-dimensional device simulator. The structural parameters of the device and bias conditions on the buried layer are investigated to understand the limitations and the potential of devices. It is found that the emitter structure should be optimized to achieve the high efficiency of current confinement and the design of overlap between base-collector junction and buried layer is effective to suppress the carrier-blocking effect. Moreover, proposed C-up HBT demonstrates the similar current-gain cutoff frequency (fT) characteristics compared with conventional C-up HBT fabricated by ion implantations. The impact of fT caused by the external base-emitter capacitance (CBE,ext) can be relieved by further structural optimization of the emitter layer and lateral scaling of the extrinsic region. To clarify the feasibility of the proposed C-up HBTs, we also specify the fabrication process for the devices with epitaxial regrowth techniques.